Node Reaper: Refactor

[eytan-avisror]

We need to refactor node reaper, it started as a simple script, but as we add more and more logic, it seems the current structure is becoming a bit flakey.

We should have a make some structural improvements.

• K8s/AWS clients should be instantiated outside, this will allow to unit tests Run()
• Better data structures for for reapable/drainable
• Remove duplicate code
• A lot of unnecessary if conditions
• Pass arguments to ReaperContext and validate them in a cleaner way

[aakashchan]

can i work on this? @eytan-avisror

[eytan-avisror]

@Chan24
Sure, but can you please keep PRs small (vs. very large refactor PRs) so we can review/merge easily?